The permeable crust conditions for a Europan biosphere

The two major terrain-forming processes on Europa are: (1) melt-through, creating chaos, and (2) tectonic processes of cracking and subsequent ridge formation, dilation, and strike-slip. These two major processes have continually destroyed preexisting terrain, depending on whether local or regional heat concentration was adequate for large-scale melt-through or small enough for refreezing and continuation of tectonism. The processes that create both chaotic and tectonic terrains (and probably cratering as well) all include transport or exposure of oceanic water through the crust to the surface.

Whether life was able to begin on Europa, or exists there now, remains unknown, but the physical conditions seem propitious (e.g., Greenberg (2005)). Change probably occurs over various timescales, which may provide reasonable stability for life, while also driving adaptation and evolution.

Cracks are formed in the crust due to tidal stress and many penetrate from the surface down to the liquid water ocean. The cracks are subsequently opened and closed with the orbit-driven tidal working of the body. Thus, on a timescale of days, water flows up to the float line during the hours of opening, and is squeezed out during the hours of closing. Slush and crushed ice are forced to the surface, while most of the water flows back into the ocean. The regular, periodic tidal flow transports substances and heat between the surface and the ocean.

At the surface, oxidants are continually produced by disequilibrium processes such as photolysis by solar ultraviolet radiation, and especially radiolysis by charged particles. Significant reservoirs of oxygen have been spectrally detected on Europa's surface in the form of H2O2, H2SO4, andCO2 (Carlson etal., 1999; McCord etal., 1998b), while molecular oxygen and ozone are inferred on the basis of Europa's oxygen atmosphere (Hall et al., 1995) and the detection of these compounds on other icy satellites (Calvin et al., 1996; Noll et al., 1996). Moreover, impact of cometary material should also provide a source of organic materials and other fuels at the surface, such as those detected on the other icy satellites (McCord et al., 1998c). In addition, significant quantities of sulphur and other materials may be continually ejected and transported from Io to Europa.

The ocean probably contains endogenic substances such as salt, sulphur compounds, and organics (e.g., Kargel et al. (2000)), as well as surface materials that may be transported through the ice. Oceanic substances most likely have been exposed as the orange-brown darkening along major ridge systems and around chaotic terrain. While the coloration displayed in images taken at visible-to-near-infrared wavelengths is not diagnostic of composition, near-infrared spectra are indicative of frozen brines (McCord et al., 1998a), as well as sulphuric acid and related compounds (Carlson et al., 1999). The orangish brown appearance at visible wavelengths may be consistent with organics, sulphur compounds, or other unknowns. The ocean probably contains a wide range of biologically important substances.

The ice at any location may contain layers of oceanic substances, which are deposited at the top during ridge formation, and then work their way deeper as the ice maintains thickness by melting at the bottom (until a melt-through event resets the entire thickness as a single layer of refrozen ocean). This process can bury surface materials, eventually feeding (or recycling) them into the ocean. For oxidants, burial is especially important to prevent their destruction at the surface, and impact gardening may help with the initial burial (Phillips and Chyba, 2001).

Chemical disequilibrium among materials at various levels in a crack is maintained by production at the top and the oceanic reservoir at the bottom, while the ebb and flow of water continually transports and mixes these substances vertically during the tidal cycle. Transport is through an ambient temperature gradient ~0.10 °C m-1, from 0 °C at the base of the crust to about 170 °C colder at the surface.

The physical conditions in such an opening in the ice might well support life as illustrated schematically in Figure 15.10. No organisms could survive near the surface, where bombardment by energetic charged particles in the Jovian magnetosphere would disrupt organic molecules (Varnes and Jakosky, 1999) within ~1 cm of the surface. Nevertheless, sunlight adequate for photosynthesis could penetrate a few metres, farther than necessary to protect organisms from radiation damage (Reynolds et al., 1983; Lunine and Lorenz, 1997; Chyba, 2000). Thus, as long as some part of the ecosystem of the crack occupies the appropriate depth, it may be able to exploit photosynthesis. Such organisms might benefit from anchoring themselves at an appropriate depth where they might photosynthesize, although they would also need to survive the part of the day when the tide drains away and temperatures drop. Other non-photosynthesizing organisms might anchor themselves at other depths, and exploit the passing daily flow. Their hold would be precarious, as the liquid water could melt their anchorage away. Alternatively, some might be plated over by newly frozen water, and frozen into the wall. The individuals that are not anchored, or that lose their anchorage, would go with the tidal flow. Organisms adapted to holding onto the walls might try to reattach their anchors. Others might be adapted to exploiting movement along with the mixing

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Jupite^s magnetosphere oxidants surface oxidants surface

Fig. 15.10. Tidal flow through a working crack provides a potentially habitable setting, linking the surface (with its low temperature, radiation-produced oxidants, cometary organic fuels, and sunlight) with the ocean (with its brew of endo- and exogenic substances and relative warmth). Photosynthetic organisms (represented here by the tulip icon) might anchor themselves to exploit the zone between the surface radiation danger and the deeper darkness. Other organisms (the tick icon) might hold on to the side to exploit the flow of water and the disequilibrium chemistry. The hold would be difficult, with melting releasing some of these creatures into the flow, and with freezing plating others into the wall of the crack. Other organisms (jellyfish icon) might exploit the tides by riding with the flow. This setting would become hostile after a few thousand years as Europa rotates relative to Jupiter, the local tidal stress changes, and the crack freezes shut. Organisms would need to have evolved strategies for survival by hibernating in the ice or moving elsewhere through the ocean. (Artwork by Barbara Aulicino/American Scientist.)

Fig. 15.10. Tidal flow through a working crack provides a potentially habitable setting, linking the surface (with its low temperature, radiation-produced oxidants, cometary organic fuels, and sunlight) with the ocean (with its brew of endo- and exogenic substances and relative warmth). Photosynthetic organisms (represented here by the tulip icon) might anchor themselves to exploit the zone between the surface radiation danger and the deeper darkness. Other organisms (the tick icon) might hold on to the side to exploit the flow of water and the disequilibrium chemistry. The hold would be difficult, with melting releasing some of these creatures into the flow, and with freezing plating others into the wall of the crack. Other organisms (jellyfish icon) might exploit the tides by riding with the flow. This setting would become hostile after a few thousand years as Europa rotates relative to Jupiter, the local tidal stress changes, and the crack freezes shut. Organisms would need to have evolved strategies for survival by hibernating in the ice or moving elsewhere through the ocean. (Artwork by Barbara Aulicino/American Scientist.)

flow. A substantial fraction of that population would be squeezed into the ocean each day, and then flow up again with the next tide.

A given crack is probably active over thousands of years, because rotation is nearly synchronous and the crack remains in the same tidal-strain regime, allowing for a degree of stability for any ecosystem or organisms within it. Over longer times, with non-synchronous rotation a given site moves to a substantially different tidal-strain regime in 103-105 y. Then the tidal working of any particular crack is likely to cease. The crack would seal closed, freezing any immobile organisms within it, while some portion of its organism population might be locked out of the crack in the ocean below.

For the population of a deactivated crack to survive, (a) it must have adequate mobility to find its way to a still active (generally more recently created) crack, or else or in addition (b) the portion of the population that is frozen into the ice must be able to survive until subsequently released by a thaw. At any given location, a melt event probably has occurred every few million years, liberating frozen organisms to float free and perhaps find their way into a habitable niche. Alternatively, in the timescale for non-synchronous rotation (less than a million years), fresh cracks through the region would cross the paths of the older refrozen cracks, liberating organisms into a niche similar to where they had lived before. Survival in a frozen state for the requisite few million years seems plausible, given evidence for similar survival in Antarctic ice (Priscu et al., 1999). The need to survive change may provide a driver for adaptation and mobility, as well as opportunity for evolution.

We have shown that this model creates environments in the crust that may be suitable for life. Moreover, it provides a way for life to exist and prosper in the ocean as well, by providing access to necessary oxidants (Gaidos et al., 1999; Chyba and Phillips, 2001) and linkage between oceanic and intracrust ecosystems. Oceanic life would be part of the same ecosystem as organisms in the crust. Components of the ecosystem might adapt to exploit suboceanic conditions, such as possible sites of volcanism. If there is an inhabited biosphere on Europa, it most likely extends from within the ocean up to the surface. While we can only speculate on conditions within the ocean, we have observational evidence for conditions in the crust, and the evidence points toward a potentially habitable setting.

This model is based on the surface manifestations of tectonic processing and chaotic terrain formation. The entire surface is very young, <2% of the age of the Solar System according to the paucity of impact craters. Because the tectonic and melting processes described above were recent, they may well continue today. Whether any organisms exist on Europa to exploit this setting is unknown.

Spacecraft exploration of Europa is likely to continue in the future. With the likelihood that the liquid ocean is linked to the surface in multiple ways, Europa's biosphere may be exposed at the surface, facilitating exploration and also contamination. Unless exploration is planned very carefully, we may discover life on Europa that we had inadvertently planted there ourselves. The advantage from the point of view of exploration is that, if landing sites are chosen wisely, it may not be necessary to drill down to the ocean in order to sample the deep. Oceanic materials, possibly including organisms, may be readily accessible at or near the surface. Thus, the search for life in that habitable zone may be less daunting than has been assumed in the past.

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